1. Field of the Invention
This invention relates to an aluminum alloy for casting purposes which is superior in castability, strength, and resistance to heat, thermal shock and permanent deformation by heat.
2. Description of the Prior Art
Aluminum has recently come to use for a wide range of applications including the manufacture of vehicles and machines. As a result, there has been a strong need for an aluminum alloy for casting purposes which is superior in strength and heat resistance.
The inventors of this invention have succeeded in developing a previously proposed (hereinafter indicated as previous alloy) casting alloy which is superior in castability, strength and heat resistance, as disclosed in Japanese Laid-Open Patent Specification No. 69234/1980. The previous alloy contains over 6% and up to 13% of silicon, over 3% and up to 5% of copper, over 0.2% and up to 1% of magnesium and over 0.03% and up to 1% of antimony, the balance being aluminum and impurities. It has a maximum strength which is as high as 40 kg/cm.sup.2, and an elongation of 3 to 4%, and is by far superior in thermal shock resistance to any conventionally known alloy of this kind**. It is, therefore, suitable for use in the manufacture of a part of a machine which repeatedly exposed to intense heat, for example, a piston in an engine. FNT **(for example, Japanese Industrial Standard for aluminum casting alloys "AC8A" and "AC8B")
Further research of the inventors has, however, indicated that the previous alloy has a number of defects. They have found that if a piston made of this alloy is used for a long time, that portion of the piston which has been exposed to heat repeatedly undergoes a permanent volumetric shrinkage which is different from that resulting from ordinary thermal expansion and contraction, and that the shrinkage enlarges the clearance between the piston and the cylinder, resulting in a blowby or piston slap. Moreover, the alloy is liable to lamellar abrasion, for example, in a groove in which a piston ring is fitted, resulting in failure of the ring to function properly.